This invention relates generally to techniques for one platform to acquire another for the purposes of establishing a communications path there between and, more specifically, relates to method and apparatus providing a spatial search pattern to enable a first terrestrially-based or airborne platform to acquire a second terrestrially-based or airborne platform for establishing a point-to-point communications path.
A problem arises when two platforms, such as two airborne platforms, are required to establish a point-to-point, line-of-sight (LOS) communications path between themselves using one or more directional antennas (i.e., where at least one antenna must be pointed at the other). In this case the two platforms may not have any a priori knowledge of the location of the other in three dimensional space, nor any knowledge of the relative heading of the other platform, nor any knowledge of the speed of the other platform. As can be appreciated, this set of conditions can severely complicate the initial acquisition phase, and can result in an inordinately long period of time where each platform searches for the other (such as by transmitting a probe or acquisition signal, and attempting to receive a corresponding probe or acquisition signal from the other platform). The initial acquisition phase can be contrasted with the subsequent tracking phase where, after the point-to-point communication path has been successfully established, the antennas of the two platforms can remain pointing at one another using conventional closed-loop feedback techniques.
While the acquisition problem can be most troublesome when the two platforms are both airborne, similar problems exist where one platform is terrestrially sited, and the other is airborne, or even when both platforms are terrestrially-based, especially in terrain characterized by changes in elevation, such as hilly or mountainous terrain. As employed herein two ships at sea are also considered to be examples of two platforms that are terrestrially-based.
While it may be possible to provide special transmitters and/or receivers (e.g., having larger beamwidths than those used for communications) to aid in the initial acquisition phase, this is an undesirable approach in that it adds cost, weight and complexity to each platform.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
In one aspect this invention provides a method for a first platform and a second platform to obtain information that is descriptive of a relative location of the other. The method includes establishing an initial antenna pointing direction of the first and second platforms such that the pointing directions are opposite one another, and incrementally scanning each antenna in azimuth in the same direction in synchronism with one another in a plane referenced to a common reference plane until each antenna is within the other antenna""s azimuth and elevation beamwidth during a scanning increment dwell time (TDWELL). Upon completing a scan in azimuth in the plane, the method changes an elevation angle of each antenna pointing direction relative to the plane by equal and opposite amounts, and repeats the incremental scanning of each antenna in azimuth in the same direction.
A further aspect of this invention provides an acquisition method for use in establishing a line-of-sight communication path between a first antenna of a first platform and a second antenna of a second platform. This method includes (a) defining a first spherical search space that is centered on the first antenna and a second spherical search space that is centered on the second antenna, each spherical search space being characterized by having lines of longitude corresponding to antenna azimuth pointing directions and lines of latitude corresponding to antenna elevation pointing directions, where an equatorial plane of each spherical search space is referenced to a plane that is tangent to the surface of the Earth; (b) establishing an initial antenna pointing direction of the first and second antennas such that the pointing directions are opposite one another referenced to an Earth-based coordinate system; and (c) operating within the spherical search space or a subset of the spherical search space by incrementally scanning each antenna in azimuth in the same direction in synchronism with one another, and upon completing a scan in azimuth, changing an elevation angle of each antenna relative to the equatorial plane in synchronism with one another, and repeating the incremental scanning of each antenna in azimuth in the same direction until each antenna is within the other antenna""s azimuth and elevation beamwidth during TDWELL.
In the presently preferred embodiment the beamwidth of the first antenna differs from the beamwidth of the second antenna, a minimum value of TDWELL is common for both antennas, and where a minimum antenna step size is a function of the smallest beamwidth.
Apparatus that operates in accordance with this invention is also disclosed, as is a computer readable media that stores computer instructions for implementing a computer program to cause the computer to execute an acquisition method for use in establishing the line-of-sight communication path between the first antenna of the first platform and the second antenna of the second platform, in accordance with this invention.